Size dependence of the magnetic properties of antiferromagneticCr2O3nanoparticles

Abstract

Magnetic properties of antiferromagnetic (AFM) ${\text{Cr}}_{2}{\text{O}}_{3}$ nanoparticles have been studied as a function of the nanoparticle size. The synthesized nanoparticles present an ellipsoidal shape with the major axis of approximately 170 nm and the minor axis that increases with the synthesis temperature from 30 to 70 nm. By magnetization and electron paramagnetic resonance experiments, we have obtained the parameters that characterize the AFM nanoparticles system. We have found that the N\'eel temperature, ${T}_{N}$, and the spin-flop field, ${H}_{\text{SF}}$, increase with the particle size from ${T}_{N}=288\text{ }\text{K}$ and ${H}_{\text{SF}}(5\text{ }\text{K})=10\text{ }\text{kOe}$ for the smaller nanoparticles and approach the bulk values [${T}_{N}=308\text{ }\text{K}$ and ${H}_{\text{SF}}(5\text{ }\text{K})=60\text{ }\text{kOe}$] for the larger particles. From the experimental results and the molecular-field theory applied to AFM coupled sublattices, we estimated the magnetic anisotropy, $K$, and the molecular-field constant, $\ensuremath{\lambda}$, as a function of the ${\text{Cr}}_{2}{\text{O}}_{3}$ nanoparticle size. When the size is reduced, $\ensuremath{\lambda}$ only diminishes $\ensuremath{\sim}8%$ with respect to its bulk value $(4.9\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }{\text{Oe}}^{2}\text{ }\text{g}/\text{erg})$; instead, $K$ decreases more than an order of magnitude from $K=3.8\ifmmode\times\else\texttimes\fi{}{10}^{4}$ to $8.7\ifmmode\times\else\texttimes\fi{}{10}^{2}\text{ }\text{erg}/\text{g}$. We analyzed the results on the basis of a core shell model where the nanoparticle internal order consists of an antiferromagnetically ordered core and a disordered surface shell, which presents a frustrated magnetic state.